1. Field of the Invention
The present invention relates to a driver circuit for outputting an output signal in response to an input signal given to the driver circuit.
2. Related Art
As a conventional driver circuit for outputting an output signal in response to an input signal given to the driver circuit, a driver circuit 400 shown in FIG. 1 has been known. Predetermined voltages of H level V1 and L level V2 are given to buffer amplifiers 402 and 404 respectively to control CMOS switches 406 and 408 in response to a given input signal, and thereby either the output of the buffer amplifier 402 or 404 is supplied to a device under test DUT via an output resistor 410 and a transmission line 412.
In such CMOS switches (406 and 408), however, there is operating voltage dependence of resistance when the switch is turned on. Therefore, in the driver circuit 400 it is difficult for the output impedance to match the characteristic impedance of the transmission line 412. Although the impedance matching can be achieved by inserting a buffer circuit that can be operating at high speed to the next stage of the CMOS switches (406 and 408), if the buffer circuit is inserted, an error in the output voltage occurs due to the voltage offset and the voltage gain in the buffer circuit.
As a circuit to eliminate such error, a driver circuit 400 shown in FIG. 2 has been known as disclosed, for example, in Japanese Patent Application Laying-open No. 1994-77784. This driver circuit 400 is provided with a buffer circuit 420 and simulating buffer circuits (422 and 424) of the same characteristics, and allows the buffer circuit 420 to simulate both states of the H and L levels using the buffer circuits (422 and 424). Both output voltages of the buffer circuits (422 and 424) are fed back towards buffer amplifiers 402 and 404, so that the output voltage of the buffer circuit 420 can be controlled highly accurately.
However, since the device under test DUT is provided with a plurality of buffer circuits (422 and 424) of the same characteristics as the buffer circuit 420 that supplies the output voltage, the power consumption of the entire driver circuit 400 increases, and the size of the circuit is also large, which is not practical.
In addition, as a driver circuit not using any buffer circuit, a driver circuit using a differential switch has been known as disclosed, for example, in Japanese Patent Application Laying-open No. 2001-57512. The driver circuit controls its output voltage more accurately by generating its output voltage by allowing a current to flow through a resistor in the differential switch part, and controlling the current flowing through the differential switch part using the voltage of a current route that simulates the states of the differential switch. Even in the above circuit, however, the current consumption in the differential switch is increased if a signal of large amplitude is generated.
As described above, in the conventional driver circuits, it is difficult to control the output voltage highly accurately due to the impedance matching, the error of the voltage offset and the voltage gain in the buffer circuits. In addition, in case of controlling the output voltage highly accurately, the power consumption is increased, and therefore there is a problem that both the low power consumption and the highly accurate output voltage contradict each other.